Method and apparatus for leaching metal from mining ore

ABSTRACT

Porous leach pipe and method for leaching metals from mining ores. The porous pipe may be buried beneath the surface of a pile of mined ore, providing a more even and uniform distribution of the leaching solution across the pile, increasing metal yields, reducing water consumption and eliminating pooling and ponding of the solution on the top of the piles as occurs with prior art drip line emitters.

The present invention relates to a method and apparatus for leachingmetal from a pile of mining ore, which method and apparatus delivers aleaching solution more uniformly across the pile thereby increasingmetal yields, reducing water consumption and reducing environmentalconcerns.

BACKGROUND

Copper, gold and other mined ores are blasted or crushed into smallchunks and placed directly into large heaps where the ore can beirrigated with a leach solution of e.g, sodium cyanide (NaCN), potassiumcyanide (KCN), or sulphuric acid (H₂SO₄), applied to the pile. Thesolution percolates through the heap, bonds to particles of metals suchas gold or copper, and the leach solution is then captured (e.g., by animpermeable plastic or clay-lined leach pad at the bottom of the pile)and processed into pure metal. The leach process through a piletypically takes several weeks. Once the metals are removed, the leachsolution is recycled and used again on the pile to leach more metal.

In the past, large irrigation sprinklers have been used to deliver theleach solution to the pile. This practice is rarely seen today becauseof the tremendous amount of water loss (e.g., evaporation and runoff)with this method.

More traditionally, the leaching solution is delivered to the pilesusing drip irrigation, i.e., nonporous tubing with drip emitters moldedinto the tubing at spaced intervals (most often at 28 inch intervals),the irrigation system being laid out across the surface of the pile.Based on their flow characteristics and periodic placement along theline, drip emitters cause a phenomenon known as channeling in the ore.Channeling occurs where a large amount of solution is dripped in onespot over time. This spot quickly saturates and the solution thenchannels into a relatively narrow stream, traveling quickly down throughthe pile as the solution seeks the path of least resistance to thebottom of the pile. Channeling produces a very uneven distribution ofthe leaching solution in the pile, with some areas extremely wet andothers dry. Gold and copper are only leached from ore that makes directand extended contact with the leach solution, and therefore in pilesusing drip emitters there are areas between the channels that do not getadequate leaching and metal removal.

Still further, because drip irrigation applies the solution to thesurface of the pile, and because a large volume of solution is drippedonto a select spot, drip emitters cause pooling and ponding of the leachsolution on the top surface of the ore pile. Both sodium cyanide andsulphuric acid can be toxic to wild life. In various jurisdictions,there is a major push for all leach lines to be buried because of thenegative environmental impact of surface drip irrigation. While buryingdrip irrigation is possible, when buried the emitters tend to clog,further reducing the efficiency of the leaching process and greatlyincreasing the amount of labor required to keep them in operation.

FIG. 1 illustrates the problems encountered with burying a standard dripline emitter below the surface of the pile. FIG. 1 is a schematic crosssection of a leach pile 5 showing a drip line 7 buried 6 to 18 inchesbelow the top surface 6 of the pile. The emitters 8 are shown in crosssection, disposed every 28 inches across a section of the pile. For easeof illustration, only a small section of the pile is shown, it beingunderstood that a typical pile may range from 300 by 300 feet in crosssectional area, increasing up to 1000 by 1000 feet in cross sectionalarea, with a starting height of 40 feet, and increasing over time to1000 or more feet high. Thus, these piles of ore are truly enormous andtheir very large scale must be taken into consideration in understandingthe problems addressed by the present invention.

On the left hand side of FIG. 1, a standard drip line 7 is shown buriedbeneath the surface of a leach pile having a relatively low clay content(i.e., less compacted than a high clay content pile). Although the leachsolution 9 can quickly saturate the area below each of the individualemitters 8, spanning out in the process, there are still large areasbetween the emitters where the amount of leaching solution is deficient,reducing the efficiency of the process. Still further, when leachemitters are buried, the emitters quickly clog with particles of ore,sand or soil, further reducing the efficiency of the leaching processand greatly increasing the amount of labor required to keep them inoperation.

On the right hand side of FIG. 1, a standard drip line 7 is shown buriedbeneath the surface of a leach pile having a high clay content, orotherwise compacted. Here the problems are even greater. The pointsource delivery of the solution is not absorbed quickly by the pile dueto its high density, and so the solution flows along the drip line 7,concentrates, and rises to the surface 6 of the pile forming a pool 9;it also discharges from the side 11 of the pile. The pooling of leachsolution on the surface leads to evaporation losses and potential injuryto wildlife.

As a result of these multiple problems with buried drip lines, the vastmajority of users continue to lay the drip line emitters on the topsurface of the pile. It is both easier and cheaper, avoiding the expenseof burying the drip line, avoiding the emitters being crushed or damagedby the process of burying them under the ore, and reducing thelikelihood that the emitters will become clogged by the particles in theore pile.

There has long been a need for improvements in the current methods fordelivering a leaching solution to a pile of mining ore in an efficientand cost effective manner. Despite such long felt need, there have beenlittle changes in the process over the past decades.

SUMMARY OF THE INVENTION

According to the present invention, a new mining leach pipe is provideddesigned to evenly deliver a leach solution at a controlled rate (e.g.,1 gallon per 100 feet per minute) over extended distances in the rangeof e.g, 150 to 300 feet. The pipe is designed to be buried beneath thesurface of a leach pile to achieve significantly higher metal yieldswhile avoiding the channeling and environmental concerns associated withthe pooling/ponding of the prior art. Still further, due to thelabyrinth of channels provided throughout the length of this porouspipe, the rate of clogging is dramatically reduced.

Thus, the present invention provides one or more of the followingadvantages:

-   -   uniform leak rates over long runs of tubing;    -   crush resistant wall that allows the pipe to be mechanically        installed using heavy equipment, and allows it to be buried in        rock/ore without crushing;    -   dramatic reduction in water consumption and runoff when compared        to surface drip emitters, as burying the leach pipe        significantly reduces evaporation associated with the        pooling/ponding of the prior art;    -   higher metal yields from the leach solution due to the more        uniform wetting of the pile by the new porous leach pipe;    -   elimination of the environmental hazards associated with the        pooling and ponding of the leaching solution according to the        prior art.

In accordance with one embodiment of the invention, a method of leachingmetal from a pile of mining ore is provided comprising:

-   -   locating a leach pipe below the surface of the pile;    -   supplying a leach solution to an inlet end of the pipe to        pressurize the pipe with the leach solution; and    -   the leach pipe comprising a flexible microporous tubular wall of        select length providing a substantially continuous and        consistent delivery rate of the leach solution along such length        as the solution seeps through the microporous wall of the        pressurized pipe.

In accordance with another embodiment of the invention, an apparatus isprovided for leaching metal from mining ore comprising a microporousleach pipe adapted to be buried beneath a surface of a pile of miningore, the microporous leach pipe comprising a flexible microporoustubular wall of rubber or plastic material having a porous sponge-likestructure with a multiplicity of interconnected irregular shaped poressuch that a leach solution under pressure in the pipe will seep throughthe wall at a rate of from 0.5 to 2.0 gallons per 100 feet per minute.

In one embodiment, the microporous wall has a pore size in a range offrom 0.001 to 0.004 inch.

In one or more embodiments, the length of the microporous wall is atleast 100 feet, from 100 to 300 feet, or from 300 to 600 feet.

In one or more embodiments, the microporous wall has a wall thickness ofat least 0.05 inches, from 0.05 to 0.5 inch, or from 0.1 to 1 inch.

In one or more embodiments, the microporous wall has an inner diameterof at least 0.25 inch from 0.25 to 1 inch, or from 0.5 to 0.75 inch.

In one embodiment, the microporous wall comprises a wall of rubber orplastic material having a porous sponge-like structure with amultiplicity of interconnected irregular shaped pores such a leachsolution under pressure in the pipe will seep through the wall at a rateof from 0.5 to 2.0 gallons per 100 feet per minute.

In one embodiment, the leach solution comprises sodium cyanide,potassium cyanide or sulphuric acid and optionally includes a pH buffer(e.g., a salt, a caustic metallic base such as sodium hydroxide).

In one or more embodiments, the delivery rate is at least 0.5 gallonsper 100 feet per minute, from 0.5 to 2.0 gallons per 100 feet perminute, from 0.8 to 1.5 gallons per 100 feet per minute, or from 0.9 to1.1 gallons per 100 feet per minute.

In one or more embodiments, the delivery rate is at least 0.005 gallonsper foot per minute, from 0.005 to 0.02 gallons per foot per minute, orfrom 0.008 to 0.015 gallons per foot per minute.

In one or more embodiments, the pressure is at least 8 psi at the inletend of the leach pipe, at least 10 psi, from 15 to 80 psi, or from 20 to50 psi.

In one or more embodiments, the pressure drop along the length of theleach pipe is from 10-60% per 100 feet, 2-3 psi per 100 feet, or 8-12psi per 100 feet.

In one or more embodiments, the leach solution is delivered for at least45 days while maintaining a delivery rate of at least 0.5 gallons per100 feet per minute.

In one or more embodiments, wherein the metal comprises at least one ofgold, copper and chromium.

In one or more embodiments, the wall comprises thermoset polymerparticles and a thermoplastic binder.

In one or more embodiments, the particles comprise rubber, natural orsynthetic rubber, reclaimed rubber, or cured crushed rubber.

In one or more embodiments, the rubber particles comprise at least 50weight percent of the pipe, from 50 to 80 weight percent, or from 60 to70 weight percent.

In one or more embodiments, the binder comprises an ethylene polymer,polyethylene, or low density polyethylene.

In one or more embodiments, the rubber particles have a fineness ofabout 5 to 200 mesh, about 10 to 100 mesh, or about 30 to 50 mesh.

In one or more embodiments, the fluid delivery rate varies by no greaterthan 10% per two foot section of a 100 foot microporous wall length, orno greater than 5% per two foot section of a 100 foot microporous walllength.

In one or more embodiments, the microporous wall has a pore size in arange of from 0.001 to 0.004 inch, a wall thickness of from 0.05 to 0.5inch, an inner diameter of from 0.25 to 1 inch, and a length of at least100 feet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a prior art drip line emitter, andthe problems associated with such drip line emitters if they are buriedbeneath the surface of the pile;

FIG. 2 is a schematic illustration of one embodiment of a porous leachpipe of the present invention, shown buried beneath the surface of apile of mining ore, illustrating the dramatically greater distributionand efficiency of the delivery process;

FIG. 3 is a longitudinal cross sectional schematic view of a section ofmining leach pipe according to one embodiment of the invention,illustrating the microporous wall structure, a coupling at the input endand a plug at the distal end;

FIG. 4 is a schematic illustration, partially in cross section, of oneembodiment of the invention showing main and secondary delivery pipelines on the surface of a mining ore pile, and sections of themicroporous leach pipe branching from the secondary delivery pipes atperiodic intervals across the pile, e.g., 2 to 3 feet intervals, theporous pipe being buried beneath the surface of the pile; and

FIG. 5 illustrates a section of the microporous leach pipe of FIG. 4 atthe point of burial, showing how the leaching solution seeps through andwets the ore around the pipe.

DETAILED DESCRIPTION

FIG. 2 is a schematic representation of one embodiment of the inventionin use, showing a porous mining leach pipe 20 buried (e.g., 6 inches to18 inches) below the surface 16 of a pile 15 of mining ore. The porouspipe, as described further below, provides a uniform and slowdistribution of leaching solution 19 along the length of the pipe. Theleaching solution seeps through a microporous wall and wets the outersurface of the pipe, in contrast to the dripping action of the prior artdrip line emitters. The seeping action more effectively soaks theadjacent earth in the mining pile, avoids pooling and increases theproportion of the ore in the pile that is in contact with (wetted by)the leaching solution over an extended period of time—a necessarycomponent of an effective and high volume leaching process. The pipe isdesigned to withstand the burying process, as well as the weight of theore piled on top of the pipe. Still further, the microporous structureof the pipe resists clogging when buried, enabling the pipe to be usedmore cost effectively and for extensively longer periods of time thanthe prior art drip emitters.

In one embodiment, the mining leach pipe is designed to evenly leakwater (leach solution) at a controlled delivery rate of approximately 1gallon per 100 feet per minute, substantially uniformly across distancesof between 150 and 300 feet. The porous pipe is crush resistant, with apreferred wall thickness in a range of 0.05 to 0.5 inch. The innerdiameter can range from 0.25 to 1 inch. The combination of uniformdelivery over long runs and slow seepage allows the leach pipe toachieve significantly higher metal yields from the pile, avoidingchanneling, while also eliminating the environmental and waterloss/evaporation problems with pooling/ponding. Also, by maintaining acontinuous porous passageway and substantially consistent pore size(e.g., in a range of from 0.001 to 0.004 inches) across the full lengthof the tubing, the rate of clogging is dramatically reduced. The porousmining pipe has a labyrinth of channels throughout its length which hasbeen found to avoid the adverse clogging effects of the prior artemitters (e.g., due to pH buffers in the leach solution, e.g., sodiumhydroxide, and/or due to the earth/ore itself e.g., calcium carbonate).

In one embodiment, the porous pipe comprises thermoset polymer particlesand a thermoplastic binder for binding the particles into a compositestructure with a substantial volume of void space (the macroporouschannels). The pipe may be formed as an extrudable mixture in which amajor portion comprises the thermoset polymer particles and a minorportion the thermoplastic binder. No further constituents are required;however it may be desirable to include small amounts of slip agents orlubricants depending upon the process parameters. Examples of suitablethermoset polymer particles include natural or synthetic rubber. Curedcrumb rubber reclaimed from the tread portions of vehicle tires, isreadily available and an inexpensive source of the major component. Therubber may be ground into crumb like particles which are of a mesh sizeof about 5 to 200 mesh, more specifically about 10 mesh to 100 mesh, andstill more specifically about 30 to 50 mesh.

The binder component may be a thermoplastic resin material such aspolyethylene (PE), and more particularly a linear low densitypolyethylene resin capable of thermal softening below about 300 degreesF., for extrusion processing with the crumb rubber particles in anextruder die that operates at a temperature ranging from about 350 to365 degrees F. Other binders may be used, however PE is preferred sinceit is generally unreactive in rock and soil environments over long-termuse, and to various chemicals that may be used in the leaching solution.Linear low density polyethylene's are known having a density rangingfrom about 0.90 to 0.93 gram per cubic centimeter, and porous pipe madefrom such binder resin is flexible and can be bent to desiredconfigurations and contours. Polyethylene may be employed in the form ofgranules or particles having a fineness of about 40 mesh (0.0185 inch)to 0.125 inch.

The mixture may comprise about 50% to 80% by weight thermoset (e.g.,crumb rubber) particles and about 50% to 20% by weight thermoplastic(e.g. polyethylene) binder resin, a particular embodiment being about66% rubber particles and 34% polyethylene binder. Other particle sizesand weight percentages can be used depending on the porosity desired,the thickness, diameter and length of the pipe, the leaching solution,the composition of the ore pile and other variables of the intendedapplication. In one embodiment of a leach pipe made from rubberparticles and polyethylene binder, the pipe has a wall thickness inrange of 0.0625 to 1.9375 inch, with 0.125 being suitable for manyapplications. The outside diameter can range from 0.25 to 2.0 inch with0.84 being preferred, and a wall thickness of 0.05 to 0.25 inch.

Typically, the thermoset particles and binder are intimately mixed priorto their introduction to the extruder, or may be delivered to theextruder through separate component hoppers. The components are mixedand heated within the extruder and passed therethrough by, e.g., asingle screw having a continuous spiral flight. The mixture is thermallyprocessed together, the binder being thermally softened and the crumbrubber particles remaining as discrete individual unmeltedirregularly-shaped crumb particles. The particles are coated in part bythe binder during the mixing action of the extruder apparatus. Theporous pipe may be formed into a variety of sizes depending on itsintended use. For example, the pipe may range from about 0.25 to 1 inchin internal diameter, with a wall thickness of about 0.03 to 0.5 inch.Optimally, the porous pipe exhibits a substantially uniform rate ofdelivery of the leaching solution along its length, preferably varyingby no greater than 10% along each two foot section of a 100 footmicroporous wall, and more preferably no greater than 5%.

A suitable extrusion apparatus for making a microporous leach pipe ofcrumb rubber particles and polyethylene according to one embodiment ofthe invention, is described in one or more of U.S. Pat. Nos. 5,811,038,4,958,770, and 5,811,164.

In an alternative embodiment, the porous leach pipe may be made of amore uniform polymer composition, such as a foamed polymer. In oneexample, the foamed polymer is a polyvinyl polymer adapted to provideboth flexibility and a suitable microporous structure. The pores may beformed with blowing agents, either chemical, thermal or physical agents.The porous flexible pipe can by extruded of any suitable plasticmaterial containing a blowing agent under required conditions oftemperature and pressure to force the heated plastic mass through asuitable tubular shaping extrusion nozzle, and to cause the blowingagent to expand when the mass leaves the nozzle or extrusion die tobring about an expansion of the plastic mass with the formation of aporous, sponge-like structure.

Thus, a wide variety of plastic materials can be used in forming themicroporous leach pipe. Plastic materials such as polyethylene,polyester resins, flexible forms of nylon, polyurethane resins or thelike may be employed. A desirable combination of pipe flexibility,mechanical strength, and weather and corrosion resistance can beobtained using flexible formulations of vinyl plastics, especiallyplasticized vinyl chloride polymers. Flexible grades of polyethylene andnylon, may be shaped into suitable tubing without use of plasticizers.

When vinyl polymers (and/or most other usable plastics) are employed toform a foamed porous tubing, it is necessary to add a blowing agent aspreviously described. However, in the case of polyurethane and othermaterials, the addition of a blowing agent may be unnecessary, expansionof the plastic being obtained by formation in situ of an expansion agentduring the fabrication process. Where a blowing agent is employed, anumber of different materials may be used. For example, an organicblowing agent, e.g., dinitroso pentamethylene tetramine, or an inorganicblowing agent such as sodium bicarbonate, ammonium carbonate, ammoniumbicarbonate or ammonium sesquicarbonate. The blowing agent is usuallyemployed in about 0.5-10% by weight of the plastic mass, the exactamount being governed by the density desired in the final tubing, andthis, in turn, being controlled to some extent by the plastic and amountof plasticizer used.

Thus, the apparatus of the present invention comprises long tubularmembers having walls made of plastic or rubber material in the form of aporous sponge-like structure which contain a multiplicity ofinterconnected irregular shaped pores of such size, distribution anddegree of interconnection that water (leach solution) under pressurewithin the pipe will slowly seep through the pipe and spread out on thesurface of the pipe, so as to gradually and gently soak the adjacentareas of the ore pile in which it is buried.

By providing a plurality of elongated pores, whose major axis is at anacute angle to the longitudinal axis of the pipe, the outer surface andpores cooperate to give an extended surface distribution of the liquiddispensed through the pipe, in contrast to concentrated needle streamflow of water from drip line emitters. In performing the extrusion, thequantity and size of the ingredients is controlled relative to theextrusion conditions, e.g., temperature, pressure and speed ofextrusion, to create walls in the tubing having a sponge-like structureof interconnected pores that create a high resistance to any highvelocity flow of the leaching solution therethrough, but still enable asubstantial amount of solution to pass per unit time. The extrudedtubing is then cut into lengths as desired, to form a porous leach pipe.A suitable coupling device is fixed to one end of the pipe (typicallydone in the field) to enable the pipe to be connected to a source (e.g.,supply pipe) of leaching solution under pressure. In one embodiment, thedistal end of the tube is closed by a cap or the like, or by fusingtogether the walls of the tube.

In one embodiment shown schematically in FIG. 3, the porous tubing 21which forms the mining leach pipe 20 contains a multiplicity of pores 22which are irregular in size and shape, but which are distributedthroughout the entire volume of the tube walls. The size, number, and tosome extent the shape of the pores 22 in the tubing can be controlled bythe mesh size of the thermoset particles and binder, in the firstembodiment, or by the polymer composition and type/amount of blowingagent employed in the second embodiment, and also by the conditions usedfor extruding the tubing, particularly the extrusion speed. Typicallythe pores are longitudinally elongated and have their major axisdisposed at an acute angle to the longitudinal axis of the tubing, i.e.,disposed so that the major axis of the elongated pores does not runnormal to the tubing walls. The outside surface of the tubing can besubstantially smooth, except for the openings formed by the pores, or itmay have a roughened surface created by wrinkles or slight protrusions.Preferably, the inner surface is substantially smooth, providing lowerresistance to transmission of the leach solution along the extended pipelength.

Although the tubing is relatively flexible, it is desirable that it notcollapse when not filled with solution. Also, it must withstand rangehandling by heavy equipment during burial, and the weight of ore piledover the tubing. Such strength against collapsing is obtained by makingthe tubing walls of a suitable thickness, e.g., about 0.05 to 0.5 inchin thickness, although thicker walls can be employed. Generally, it isdesirable not to have the wall thickness less than about 0.1 inch inorder to provide the pipe with sufficient mechanical strength, and alsoto obtain proper control of liquid through the tubing walls. Generally,the pipe may have an inner diameter of about 0.375 to 1 inch, althoughsmaller or larger pipe sizes can be made in accordance with theinvention.

As shown in FIG. 3, the pipe 20 can be formed with a common circularcross section for optimizing mechanical strength. However other crosssectional shapes are possible, such as an elliptical cross section.

A coupling or fitting 30 is required at one end of the tubing 21 inorder that the soaking hose may be connected to a source of water underpressure. A variety of source feeds and coupling devices can beemployed. In one embodiment, a tubular supply line about 8 inches indiameter supplies leaching solution to a plurality of microporous leachpipes, which branch off from the supply line at regular or irregularintervals. The supply line can be, for example, high density PE or PVC.Holes may be drilled into the supply line at intervals, and acompression fitting inserted into each hole, for attachment of a porouspipe branch. In one example, the compression fitting 30 has anexternally threaded front end 31 which is screwed into a hole in thesupply line or other source of leaching solution; a nut seals theconnection. A second end 34 of the fitting has a barbed nipple whichslides inside a proximal (input) end 25 of the porous tubing 21 and acompression nut 32 is screwed to pinch the tubing against the fitting inliquid-tight communication.

In order to hold water within the pipe under pressure so that it will beforced through the pores 21, it is necessary to provide some closure orplug cap 40 at the distal end 27 of the pipe. Alternatively, thisclosing can be accomplished by forming a closed distal tip on the tube,e.g., by folding together the walls of the tubing and wrapping a wire tohold the end closed. Alternatively, the distal end of the porous tubingmay be connected to another supply line or looped back to connect to thesame supply line; in this embodiment the porous pipe is pressurized fromboth ends and there is no closed distal end.

FIG. 4 illustrates one embodiment of the invention wherein a main sourceor feeder pipe 52 supplies a plurality of secondary feeder pipes 50 a,50 b, 50 c branching off at extended intervals (e.g, 150 feet) from themain feeder pipe 52. In this example, a plurality of porous of leachpipes 55 (e.g., each about 150 feet in length) branch off from eachsecondary feeder pipe 50 at intervals of e.g., 2 feet. The feeder pipes52 and 50 are made of high density polyethylene (HDPE) with heat weldedbutt joints, the main feeder pipe 52 having e.g., a 12 inch outerdiameter OD and the secondary feeder pipes 50 each having e.g., an 8inch OD. The porous leach pipe 55 is a crumb rubber/PE composite ofabout 66 weight percent rubber and about 34 weight percent LDPE, and hasa ⅝ inch ID and 0.115 inch wall thickness. The relative internaldiameters of the feeder pipe and branching porous leach pipes, and therelative lengths and the number of branching points, will vary basedupon the particular application. As shown here, the leach solution maybe supplied to a main trunk line (e.g., 12 inch OD) which then splitsand feeds multiple secondary feed lines each having a smaller diameter(e.g., 8 inch OD) to maintain pressure along the length of the supplyline.

FIG. 4 shows in the foreground (delineated by dashed line 54) a crosssection down through the ore pile 57. Each porous leach pipe 55extending from feeder pipe 50 a is buried near (e.g, within 1-3 feet)its branch point; a cross section through each buried leach pipe 55 isshown at 56, beneath the surface 54 of the pile. The leach solution 58which has seeped from the plurality of leach pipes 55 along the lengthof feeder pipe 50 a has substantially uniformly wetted the ore acrossthe pile 58, as shown by the even shading in the foreground of FIG. 4.

One typically seeks to avoid turbulent flow in the porous leach pipes,thus utilizing a lower input pressure to hydrologically pressurize theporous pipe before it starts seeping. In other words, the leachingsolution travels from the input end (connected to the delivery pipe) toits terminal end in order to pressurize the leaching solution in theporous pipe. There will then be seepage of the leaching solution alongthe length of the pressurized porous pipe, even with the pressure dropexperienced along the pipe length, as long as the pipe remainspressurized. For example, with an input pressure of leaching solution ofabout 10 psi, a ⅝ inch ID porous pipe may experience a 2 to 3 psipressure drop per 100 feet of porous pipe. In another example, with aninput pressure of 40 to 45 psi, the pressure drop may be 8 to 12 psi per100 feet of porous pipe. With a 15 psi input pressure, and a 200 footlength porous pipe, the pressure may ultimately drop to 3 psi at aterminal end of the pipe, but still provide sufficient pressurization toperform (provide uniform seeping) along the length of the pipe.Generally, the longer the run of porous pipe, the lower the flow needsto be, i.e., so that the leaching solution does not rapidly seep out atthe initial portion of the pipe. By way of example, for a 300 footlength of porous pipe, the delivery rate may be 0.5 gallons per minuteper 100 feet. With a much shorter porous pipe length of 100 feet, thedelivery rate may be 1 gallon per minute per 100 feet. Those skilled inthe art can determine an appropriate balance of parameters for aparticular application.

FIG. 5 is a close-up view of one of the porous pipe branches 55 of FIG.4, at the point near where it enters (is buried under) the ore pile. Theleaching solution has seeped through and wet the ore in an area adjacentthe pipe, and particles of ore and soil (clay) are shown clinging to thepipe. However, there is no concentrated dripping, pooling or channelingas encountered with the prior art drip emitters. Instead, the slowseepage provides a more uniform wetting across the pile for moreefficient leaching of metal from the pile.

As used herein, the leaching of “metal” is meant to include leaching ofmetallic compounds, e.g., metals in their oxide form, as well as metalsin their pure metallic state. Some metals exist in nature only in theiroxide form and further refining operations are required to reduce themto their pure metallic state, after they have been leached/separatedfrom the ore.

While specific embodiments of the present invention have been shown anddescribed, it will be apparent that many modifications can be madethereto without departing from the scope of the invention. Accordingly,the invention is not limited by the foregoing description.

1-20. (canceled)
 21. An apparatus for leaching metal from mining ore,comprising a microporous leach pipe adapted to be buried beneath asurface of a pile of mining ore, the microporous leach pipe comprising aflexible microporous tubular wall of rubber or plastic material having aporous sponge-like structure with a multiplicity of interconnectedirregular shaped pores such that a leach solution under pressure in thepipe will seep through the wall at a rate of from 0.5 to 2.0 gallons per100 feet per minute.
 22. The apparatus of claim 21, wherein themicroporous wall has a pore size in a range of from 0.001 to 0.004 inch.23. The apparatus of claim 21, wherein the length of the microporouswall is at least 100 feet.
 24. The apparatus of claim 21, wherein themicroporous wall has a wall thickness of at least 0.05 inches.
 25. Theapparatus of claim 21, wherein the microporous wall has an innerdiameter of at least 0.25 inch.
 26. The apparatus of claim 21, whereinthe microporous wall has a pore size in a range of from 0.001 to 0.004inch, a wall thickness of from 0.05 to 0.5 inch, an inner diameter offrom 0.25 to 1 inch, and a length of at least 100 feet.